Why we need Secondary Logon Service?

Windows secondary logon allows administrators to log on with a non-administrative account and still be able to perform administrative tasks (without logging off) by running trusted administrative programs in administrative contexts. In this scenario, system administrators require two user accounts: a regular account with basic privileges, and an administrative account (this can be a different administrative account for each administrator or a single administrative account shared among administrators). 

Secondary logons address the security problems presented by administrators running programs that may be susceptible to "Trojan Horse" attacks (such as running Microsoft Internet Explorer in the administrative context while accessing a non-trusted Web site).

Even though secondary logon is primarily intended for system administrators, it can be used by any user with multiple accounts to start programs under different account contexts without the need to log off.

The Secondary Logon service is started automatically after a "clean" installation of Windows. A "clean" installation of Windows means either installing Windows on a blank hard disk, or installing Windows in a folder other than the folder in which an existing version of Windows is installed.

For More Info Refer KB:-
http://support.microsoft.com/kb/225035